1. Field of the Invention
The present invention relates generally to fluid cylinders and more particularly to self-lubricating fluid springs used between die members of a stamping press.
2. Description of the Prior Art
The service life and reliability of a nitrogen die cylinder is directly related to the ability of the high-pressure dynamic seal to maintain minimum leakage. The ability of the high-pressure dynamic seal to maintain its proper function and operation has been found to be greatly affected by the amount and the temperature of the lubricating oil film over which the seal travels. Surface contact wear and therefore leakage of the seal occurs rapidly when the seal is allowed to travel against a cylinder wall lacking an adequate film of lubricating oil.
Most nitrogen die cylinders receive film of lubricating oil on the cylinder walls during assembly. For non self-lubricating cylinders, this film of lubricating oil is sometimes supplemented by installing an oil soaked foam wiper in a groove next to the seal. However, with time, gravity tends to pull the oil film off of the cylinder. The seal itself, on each stroke, will push excess oil off the cylinder wall making the film even thinner after each stroke. The oil soaked film wiper can maintain the oil film only for a limited time before gravity and the wiping action of the seal displace the oil to areas in the nitrogen die cylinder where the oil no longer is of any benefit.
For this reason, it is preferable for such fluid cylinders to be self-lubricating. U.S. Pat. No. 4,691,902 to Kadis and U.S. Pat. No. 4,815,718 to Kadis disclose die cylinders that are very effectively self-lubricating.
In the '902 and '718 Kadis patents, the piston and the cylinder walls define a working area, and below the working area is a well. An expansion reservoir is provided outside of and around the working area and well. A separate endcap is used to form a lower portion of the cylinder body, so that the endcap can be attached to and detached from the remainder of the cylinder body, such as by threaded engagement. A first passage is machined within the endcap in which the first passage has an inlet that opens to the well. Connected to the end of the first passage opposite to the inlet is a standpipe, which then extends outward into an upper end of the expansion reservoir. Using this configuration, the inlet of the first passage and the outlet of the standpipe are the path of communication for the lubricating oil between the working chamber/well and the expansion reservoir. When the die cylinder is in an upright orientation, lubricating oil collects along the bottom of the well and in the first passage. In the upright orientation, the inlet of the first passage is disposed below the level of the lubricating oil in the well and the outlet of the standpipe and thus provides the path of communication for the lubricating oil.
When the die cylinder of the '902 and '718 patents is in an inverted orientation, the lubricating oil collects along an opposite end of the expansion reservoir and an end of the working area opposite to the well. In this inverted orientation, the outlet of the standpipe is below the level of the lubricating oil and thus acts as an inlet for the lubricating oil to be drawn through. Therefore, the first passage is the path of communication for the lubricating oil between the working chamber/well and the expansion reservoir regardless of the orientation of the die cylinder.
The die cylinder of the '902 and '718 Kadis patents also has a second passage between the working chamber and the expansion reservoir. In both the upright orientation and the inverted orientation of the die cylinder, the second passage lies above the level of the lubricating oil. Therefore, the second passage is the path of communication for the working fluid between the working chamber/well and the expansion reservoir regardless of the orientation of the die cylinder.
As with any article of manufacture, it is advantageous to simplify the design of the die cylinder, reducing the number of parts involved, while maintaining the functionality of that die cylinder. Likewise, it would be advantageous to develop the cylinder that eliminates the need for separately manufacturing and attaching that endcap to the cylinder body, and machining a passageway through the endcap, while maintaining the functional benefits of the die cylinders disclosed in the '902 and '718 Kadis patents.